This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / The 39 Clues
Cahills Vs. Vespers, big time.
The biggest one so far is from Vespers Rising, where an Elite Mook by the name of Bruno shoots the pilot of a helicopter and kicks him out of it to his presumed death. When the helicopter's thousands of feet above the ground. Yikes.
The trapeze artist working for the Vespers who was killed via lethal injection and died in agony in front of the Cahills.
Honestly, just the psychological trauma that both Cahill siblings have undergone due to the Clue Hunt and the Vesper situation. Dan is certainly not the same happy-go-lucky kid we met in Maze of Bones, and even though Amy appears to have gotten off better than he has, then she goes into Heroic Safe Mode at the end of Trust No One. Poor kids.
Day Of Doom is notorious for this, especially The Doomsday Device, which presumably had enough power to end the world. Isabel, who is Vesper Two, tries to use the device against Vesper One so that he would die. However it backfires and ends up killing her own daughter and herself...